Process for the preparation of bromochloro-alkanes

ABSTRACT

Bromochloro-alkanes are made by reaction of a corresponding chloroalkene with hydrogen bromide by a continuous process carried at -20° to +20° C., preferably in a gas-lift reactor.

This invention relates to the preparation of bromochloro-alkanes, andespecially 3-bromo-1-chloropropane.

3-Bromo-1-chloropropane and related bromochloro alkanes may be made bythe reaction of hydrogen bromide with allyl chloride or other alkenylchloride in the presence of a free radical-generating catalyst.

For example, U.S. Pat. No. 4,049,516 (Gellato et al) describes acontinuous process for the preparation of 3-bromo-1-chloropropane.Hydrogen bromide and allyl chloride are injected into a reactorcontaining 10 mole % of allyl chloride and 90 mole % of3-bromo-1-chloropropane maintained at 20°-22° C. The reaction ispromoted by irradiation of the reaction mixture with a 3050 A° lamp.Reaction product is continuously withdrawn in the form of a mixturecontaining, in addition to the desired product, also 10% of allylchloride and by-products. The % conversion of the allyl chloride isgiven as 90% and the % selectivity is given as 97%. The yield of3-bromo-1-chloropropane is thus 87.3%, based on the allyl chloride.

East German specification 74537 (Konig et al) describes a batchwise orcontinuous process for producing 3-bromo-1-chloropropane by reaction ofallyl chloride with hydrogen bromide in the presence of a peroxidecatalyst. The reaction is performed initially at about 35° C. and then,after half the allyl chloride has reacted, at about 60° C.

We have now devised a continuous process for the preparation of3-bromo-1-chloroalkanes by reaction of hydrogen bromide with acorresponding chloro-alkene in the presence of a free radical catalyst.The new process gives an excellent yield of the desired productcontaining reduced amounts of impurities such as dimerized products andother products of elevated molecular weight. It is moreover well adaptedto automated control.

The process of the present invention for the continuous preparation of a3-bromo-1-chloroalkane of formula ##STR1## where R is hydrogen or alkylof 1 to 4 carbon atoms, comprises (1) feeding into a first zone achloro-alkene of formula ##STR2## where R is hereinbefore defined, andrecycled reaction products; (2) feeding the mixture obtained into asecond, reaction zone and injecting hydrogen bromide, and, if required,a free radical reaction initiator into the said mixture in the said zonein such a manner that the mixture is vigorously agitated by the injectedgas; (3) feeding the gas-containing mixture to a third, cooling zone inwhich the mixture is cooled to maintain its temperature in the range-20° to +20° C., unreacted gas is separated, and part of the reactionmixture is withdrawn for separation of the desired product; and (4)recycling the remainder of the reaction mixture to the said first zone.

The process is preferably operated as a gas lift process with thesecond, reaction zone constituting the up-leg of the gas lift reactorand the conduit through which the remainder of the reaction mixture isrecycled to the first zone constituting the down-leg of the reactor. Useof a gas lift reactor has the advantage that relatively large amounts ofstarting materials can be treated in a reactor of small volume, and thatthe ratio of heat exchange surface to liquid volume may be kept high.Also, as explained in detail below, the process of the invention canreadily be controlled when it is operated in a gas lift reactor in theway described.

Optimum yields are obtained when the reaction temperature is controlledwithin the range 5° to 10° C.

The free radical reaction initiator used to initiate the reactionbetween the hydrogen bromide and the chloroalkene is preferably anorganic peroxide, e.g. benzoyl peroxide, but other free radicalgenerating catalysts can, if desired, be used. The free radical reactioninitiator is preferably introduced into the reactor in solution in theallyl chloride or other chloro-alkene.

It has been found that, once the reaction has been initiated by the useof a peroxide, typically at a concentration of about 0.1% by weightbased on the weight of the chloro-alkene, or other catalyst, thereaction rate is preferably maintained by injection of oxygen into thereaction mixture along with the hydrogen bromide. Since the injectedoxygen becomes partly (e.g. to the extent of about 70%) incorporatedinto the reaction products, the amount injected should be the minimumcompatible with maintaining the desired high reaction rate. Preferablythe proportion of oxygen injected is in the range from 0.3 to about 1%by volume based on the total volume of gases introduced into thereaction zone.

Hydrogen bromide is often made on a commercial scale by combustion ofbromine in hydrogen. The hydrogen bromide obtained in this way usuallycontains a small amount of unreacted hydrogen, typically about 8 volume%. The presence of this hydrogen, which does not take part in thereaction, is not injurious, but it increases the amount of reactants andproducts which leave the reactor in the unreacted gas stream, and itspresence may assist in promoting rapid contact between the hydrogenbromide and the chloro-alkene.

To ensure that all the chloro-alkene reacts, a small excess of hydrogenbromide is preferably used, e.g. about 5% in excess of thestoichiometric amount based on the chloro-alkene fed to the first zone.

The reaction proceeds very rapidly and with evolution of heat. It ishighly desirable that the reaction shall be substantially complete inthe reaction zone, and vigorous agitation of the reaction mixture in thereaction zone is therefore required. This is assisted both by theinjection of the gaseous hydrogen bromide and oxygen into the reactionzone and also, preferably, by the use of static mixers to promotethorough mixing of all the reagents. When, as is preferred, the reactionzone constitutes the up-leg of a gas-lift reactor operated in bubbleregime to maximize gas-liquid contact, the upward passage of thereaction mixture and gases through the reaction zone is promoted andmaintained by (a) the lower density of the mixture in the up-leg (whichcontains dispersed gas) as compared with the reaction product in thedown-leg, and (b) the somewhat higher temperature and consequent lowerdensity of the mixture in the up-leg as compared with the product in thedown-leg. As described below, this temperature differential can be usedin monitoring and controlling the reaction.

In the third zone, the reaction products are cooled and unreacted gas isseparated and removed. Part of the reaction mixture is then withdrawnfor separation of the desired bromochloro-alkane product. The remainderis recycled to the first zone, preferably through the down-leg of thegas-lift reactor when one is used.

The desired bromochloroalkane may be isolated from the reaction productsin known manner, typically by washing with caustic soda solution andwater followed by fractional distillation.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing shows a gas-lift reactor constructed foroperation of the new process. Use of the apparatus will be described forthe reaction of allyl chloride with hydrogen bromide to produce3-bromo-1-chloropropane, but it will be understood that other chloroalkenes can, if desired, be used in place of the allyl chloride.

In operation of the gas-lift reactor, allyl chloride is introduced intothe first zone (1) where it is mixed with reaction product entering zone(1) through down-leg (4). The ratio of the rate of feed of fresh allylchloride to zone (1) to the rate at which the recycled reaction productenters zone (1) through down-leg (4) is low, e.g. less than 1% on aweight basis. Because of the high liquid recirculation rate caused bythe presence of a gas-liquid mixture in the up-leg where an exothermicreaction takes place, the gas-lift reactor acts as a continuouslyoperated, stirred tank reactor. On an industrial scale, an injectionrate of, for example, 200 to 300 kg/hr of allyl chloride may easily beachieved, with a recirculation rate of up to 40 m³ /hr.

The mixture of allyl chloride and recycled reaction product is thenintroduced into reaction zone (2) where it is mixed with the injectedhydrogen bromide gas (optionally containing hydrogen) and the smallamount of gaseous oxygen required to maintain the reaction rate. Theweight of hydrogen bromide introduced is preferably slightly in excessof the stoichiometric amount, e.g. about 5% in excess, required to reactwith all the allyl chloride (or other chloro alkene) fed to the reactionzone.

The reaction mixture containing reacting allyl chloride and hydrogenbromide rises through the up-leg (2) of the gas lift apparatus into thecooling zone (3) where it contacts the cooling device (5). Unreacted gasrises to the surface of the liquid in zone (3) and is removed throughthe indicated gas exit. Reaction mixture is removed from zone (3) forworking up at a rate such that the mean residence time of the reactorcontents is 1.5 to 3.5 hours. Typically this involves removing from zone(3) in each hour 30-60% by weight of the total reactor contents. Theremainder is recycled through down-leg (4) as already noted.

The cooling zone (3) is preferably of such a size that in normaloperation it is about half full of liquid reaction products. This makesit possible to raise the level of the liquid, thus providing morevigorous cooling conditions, if the monitoring of the reactiontemperature indicates that more cooling is required. The rate of coolingcan also be controlled by controlling the rate of flow of the coolingfluid.

The reaction may be monitored in the following ways.

(1) The rate of circulation of the cooling fluid in the cooling zone iscontrolled by the reaction temperature (TR in the Figure) which ispreferably set at 10° C. At temperatures higher than this the coolingfluid flow is increased and at lower temperatures it is reduced. Also,as noted above, the height of the liquid in the cooling zone may becontrolled to increase or reduce the rate of cooling.

(2) The temperature difference between the up-leg (the reaction leg) andthe down-leg (the recirculation leg) is continuously monitored. Anadequate temperature difference must be maintained in order to ensurethat the reactants and products circulate normally in the gas-liftreactor. If the difference in temperature is too low, this indicatesthat the reaction is not proceeding rapidly enough, and the reactionrate can be increased by injecting more oxygen into the reaction zone.

(3) The density of the reaction products in the down-leg is continuouslymonitored. Measurement of this density makes it possible to check (a)that there are no entrained gases in the reaction products in thedown-leg, and (b) that the content of unreacted allyl chloride in thereaction products is sufficiently low. Since both the presence of gasand the presence of unreacted allyl chloride lower the density of thereaction products, it is thus a question of ensuring that the density ofthe reaction products in the down-leg is sufficiently high. Should itfall below the desired value, either the reaction rate may be increasedto remove residual allyl chloride, e.g. by injection of oxygen, or theresidence time of the reaction products in the cooling zone (3) can beincreased to permit a longer period for the gases to leave the reactionmixture. The reaction products withdrawn from the cooling zone (3)consist mainly of the 3-bromo-1-chloroalkane with a minor amount ofunreacted allyl chloride, typically equal to about 1% by weight, andsmall amounts of by-products. The following results obtained inlaboratory scale operation compare the gas-lift process of the presentinvention with a reaction carried out in a stirred tank reactor. Thefollowing Table gives the reaction temperature and mean residence timeof reagents in the reactor, and the yields of the products obtained. [Inthe Table, 2,1-BCP is 2-bromo-1-chloropropane; 1,3-BCP is1-bromo-3-chloropropane; DBP is dibromopropane; DBCP isdibromochloropropane.]

                                      TABLE                                       __________________________________________________________________________                Mean Yield on                                                                           Composition of the product obtained (Weight %)               Temperature                                                                          residence                                                                          allyl                                                                              Light                      Heavy                        Reactor                                                                            (°C.)                                                                         time (h)                                                                           chloride                                                                           Compounds                                                                           2,1-BCP                                                                            1,3-BCP                                                                            DBP                                                                              DBCP                                                                              Dimer                                                                             compounds                    __________________________________________________________________________    Stirred                                                                            25     4    93.0 0.25  4.65 93.0 0.15                                                                             0.45                                                                              0.30                                                                              0.25                         tank                                                                          reactor                                                                       Gas-lift                                                                           25     2    91.5 0.15  4.5  93.0 0.05                                                                             0.15                                                                              0.35                                                                              0.20                         recycle                                                                            13     2    93   0.50  3.9  94.1 0.05                                                                             0.10                                                                              0.25                                                                              0.15                         factor                                                                              9     2    94   0.10  3.7  94.7 0.05                                                                             0.10                                                                              0.25                                                                              0.10                               1     3.5  94   0.45  3.4  94.4 0.05                                                                             0.10                                                                              0.30                                                                              0.30                         __________________________________________________________________________

These results show that the process of the present invention, operatedat temperatures in the range of about 0° to 15° C., gives a3-bromo-1-chloropropane product which is significantly purer than theproduct obtained either with a stirred tank reactor or in a gas-liftreactor at 25° C. It will be noted also that the content of dimer andheavy compounds is significantly less with the process carried out inaccordance with the present invention.

What is claimed is:
 1. Continuous process for the preparation of a3-bromo-1-chloro-alkane of formula: ##STR3## where R is hydrogen oralkyl of 1 to 4 carbon atoms which comprises (1) feeding into a firstzone a chloro-alkene of formula: ##STR4## where R is as hereinbeforedefined, and recycled reaction products; (2) feeding the mixtureobtained in to a second reaction zone and injecting gaseous hydrogenbromide in such a manner that the mixture is vigorously agitated by theinjected gas; (3) feeding the gas-containing mixture to a third, coolingzone in which the mixture is cooled to maintain its temperature in therange -20° to +20° C., unreacted gas is separated and part of thereaction mixture is withdrawn for separation of the desired product; and(4) recycling the remainder of the reaction mixture to the first zone.2. A process according to claim 1 in which R is hydrogen.
 3. A processaccording to claim 1 in which the reaction temperature is 5° to 10° C.4. A process according to claim 1 in which the amount of hydrogenbromide injected is about 5% in excess of the stoichiometric amount,based on the chloro-alkene feed to the first zone.
 5. A processaccording to claim 1 in which the reaction is performed at ambientpressure.
 6. A process according to claim 1 in which oxygen is fed tothe reaction zone, the volume of oxygen being about 0.3 to 1% based onthe total volume of gases introduced.
 7. A process according to claim 1in which the process is operated in a gas-lift reactor, the reactionzone constituting the up-leg of the reactor and recycling conduit fromthe cooling zone to the first zone constituting the down-leg of thereactor.
 8. A process according to claim 1 wherein, in step (2), a freeradical reaction initiator is injected into the mixture in the zone. 9.A process according to claim 8 in which R is hydrogen.
 10. A processaccording to claim 8 in which the reaction temperature is 5° to 10° C.11. A process according to claim 8 in which the amount of hydrogenbromide injected is about 5% in excess of the stoichiometric amount,based on the chloro-alkene fed to the first zone.
 12. A processaccording to claim 8 in which the reaction is performed at ambientpressure.
 13. A process according to claim 8 in which oxygen is fed tothe reaction zone, the volume of oxygen being about 0.3 to 1% based onthe total volume of gases introduced.
 14. A process according to claim 8in which the process is operated in a gas-lift reactor, the reactionzone constituting the up-leg of the reactor and recycling conduit fromthe cooling zone to the first zone constituting the down-leg of thereactor.